Radiotherapy is the process of treating a tumour or other lesion (hereinafter referred to as a “tumour”) by directing a beam of harmful radiation, usually ionising radiation, such as an x-ray or electron beam, towards the lesion. The beam can be produced by an electron gun such as a linear accelerator, which produces a beam of high-energy electrons (typically in the 2-5 MeV range) which may be directed toward the patient or toward an x-ray target in order to produce an x-ray beam. A flattening filter can be inserted into the beam in order to produce a more even illumination across the cross-section of the beam.
Clearly, the beam has the potential to cause harm to the normal healthy tissue around the lesion, as well as to the tumour itself. It is therefore normal to collimate the beam so that the dose delivered to healthy tissue is minimised whereas the dose delivered to the tumour is maximised—or at least optimised, as there may be a need to limit the overall dose delivered in any one session in order to avoid necrosis and other potential complications. The direction from which the beam is delivered to the tumour can also be varied by placing the source on a gantry that is rotatable around the patient, so that different volumes of healthy tissue are in front of or behind the tumour at any one time, and this the time for which the additional dose is delivered to any particular region of healthy tissue is minimised.
Many types of collimator are available, in particular the “multi-leaf collimator” which comprises a large number (typically 40, 80 or 160) of leaves, each of which is long and thin but relatively deep in the beam direction. These are disposed adjacent each other with their long edges projecting into the beam from one side, and can be moved independently of each other into and out of the beam field. The tips of the leaves thus define an edge whose shape can be varied at will by moving individual leaves into or out of the beam.
There are various approaches to using the above arrangements to deliver a beam. Rotational conformal therapy, for example, involves rotating the radiation source around the tumour during the treatment while adjusting the multi-leaf collimator so that the cross-sectional shape of the beam matches the projected shape of the tumour along the instantaneous direction of the beam. Intensity modulated radiation therapy uses mathematical methods which start from a segmented volume identifying regions that are within the tumour (together with a desired dose), regions that are outside the lesion, and regions into which dose must be minimised, and a definition of the apparatus capabilities, and derives a treatment plan involving rotation of the gantry, collimator shapes, and dose rates which delivers a three-dimensional dose distribution which satisfies the various constraints.
All delivery methods share a common need to know the current shape and location of the lesion. However, this changes with time and between treatments. As the tumour reduces in volume in response to the treatment, it may move and allow other organs that it had displaced to return towards their usual positions. Generally, organs in the abdomen are also apt to move over time in any case, especially those below the diaphragm. At the simplest level, the patient may move during the treatment, or may be placed on the apparatus in a slightly different position or pose.
Typically, radiotherapy is delivered in a series of individual doses on a regular (e.g. daily) basis—usually referred to as “treatment fractions” or just “fractions”. To account for changes in the tumour position or shape between fractions, i.e. “inter-fraction motion”, a diagnostic image is taken immediately prior to treatment and the current position and/or shape of the tumour is determined. This is then used to adjust the treatment plan as necessary. The diagnostic image may be one or more x-ray images, or a computed tomography (“CT”) scan. Such diagnostic imaging needs a lower energy x-ray source in order to provide high quality images, typically in the range of up to 125 keV, rather than the high-energy (5 MeV) beam used for treatment which can be used for imaging, but provides very poor contrast between human tissue types. Often, a low-energy diagnostic source is provided on the same gantry in combination with the therapeutic source in order to allow for this. As the gantry is rotatable around the patient in order to allow for irradiation from multiple directions, this rotation can be used to allow the diagnostic source to develop a cone-beam CT (“CBCT”) reconstruction. Usually, the diagnostic source is located on the gantry 90 degrees from the therapeutic source, so that with the associated imaging panels for each source opposite the respective source, the items on the gantry are spaced apart and access is maximised.
It is possible to use the therapeutic beam to obtain images of the patient during treatment, a so-called “portal image”. However, as noted the image quality is poor due to a marked lack of contrast. Generally, this is adequate to confirm the gross positioning of the patent only.
To control for movement of the patient during a fraction, i.e. “intra-fraction motion management” (“IFMM”), it is more usual to attempt to fix the patient in position. The patient can be placed in an individually-tailored shaped cushion in order to ensure consistent positioning on the patient table and to limit movement during a treatment fraction, as for example disclosed in our application WO2009/006925. Restraints may be provided in order to limit movement of the patient and/or urge internal organs into a consistent position, such as is for example shown in our application WO2008/040379. For radiotherapy of the head region, a frame may be attached directly to the skull and used to fixate the head in a reproducible position. Moulded face masks can also be used to place the patient's head in a reproducible position; this is less accurate than a frame but much less invasive.
Some efforts are made to detect and respond to changes in the patient position, such as reflective markers attached to the exterior of the patient which can be detected visually. However, these are an indirect measure of the tumour position and hence of lower accuracy.
U.S. Pat. No. 7,227,925 discloses a radiation therapy treatment machine that has a stereoscopic imaging system, which includes a rotatable open gantry on which is placed a therapeutic radiation source between two diagnostic radiation sources, each with an associated diagnostic imager. The images from the two diagnostic sources are combined to produce a stereoscopic image that has location and depth information, which is used to guide the therapeutic source. In order to create a good stereoscopic image, there needs to be two diagnostic sources, placed one either side of the therapeutic source, ideally symmetrically.